The present application is based on and incorporates herein by reference Japanese Patent Application No. 2001-146178 filed on May 16, 2001.
1. Field of the Invention
The present invention relates to a monitoring system for a microcomputer which calculates the electrical angle of a rotator.
2. Related Art
The electric power steering system of a vehicle applies an assist current to a motor based on a torque signal received from a torque sensor. The torque signal indicates torque applied to the steering wheel. The motor is driven by the assist current so as to provide power assist for rotation of the steering wheel. Thereby the driver can steer the vehicle well.
The microcomputer of the electric power steering system includes an electrical angle calculator for calculating the electrical angle of the motor. The calculated electrical angle is fedback so as to be used for controlling the drive circuit for the motor. However, the microcomputer sometimes miscalculates the electrical angle due to the ambient temperature and/or noises. In this case, the amount of the assist current to be applied to the motor is controlled based on the inaccurate feedback electrical angle. As a result, the auxiliary steering force is supplied excessively or insufficiently. Then the driver cannot steer the vehicle well.
In order to overcome this problem, it is proposed that the power steering system includes a monitoring system for the electrical angle calculator. The monitoring system calculates the electrical angle of the motor independently, and then compares the calculated electrical angle with the electrical angle calculated by the electrical angle calculator. If the duration of the disagreement between the electrical angles reaches a predetermined length, it is determined that a failure occurs in the microcomputer. Then the motor is stopped and the microcomputer is stalled.
Two types of monitoring systems are proposed. One includes a supervisory circuit for monitoring a watchdog (WD) timer of the microcomputer from the outside. The other includes a supervisory microcomputer other than the microcomputer that controls the drive circuit for the motor, and thereby calculates the electrical angle of the motor redundantly. The failure in the microcomputer is detected by comparing the electrical angles calculated by the microcomputer and the supervisory microcomputer. In the case of the former monitoring system, the supervisory circuit cannot detect a failure in the microcomputer if the WD timer cannot detect the failure. In the case of the latter monitoring system, costs are high because the microcomputers should be redundantly prepared.
The present invention has an object to provide a simple and cost-effective monitoring system for detecting a failure in a microcomputer which calculates the electrical angle of a rotator.
A monitoring system according to the present invention includes a motor for driving a rotator, a rotation angle sensor for detecting the angle of the rotator, a microcomputer, supervisory signal generator means, comparator means and fault detector means. The microcomputer includes angle calculator means and control signal generator means. The angle calculator means calculates the angle of the rotator based on an excitation signal provided for the rotation angle sensor and an output signal from the rotation angle sensor. The control signal generator means generates an electrical control signal that indicates a first angle range to which the angle calculated by the angle calculator means belongs.
The supervisory signal generator means generates an electrical supervisory signal that indicates a second angle range to which the angle detected by the rotation angle sensor belongs based on an intermediate signal obtained as a result of multiplying the angle detected by the rotation angle sensor by an excitation signal provided for the rotation angle sensor. The comparator means compares the first angle range indicated by the control signal with the second angle range indicated by the supervisory signal. The fault detector means determines that a failure occurs in the microcomputer when the comparator means determines that the first angle range disagrees with the second angle range.
Preferably, the microcomputer is rebooted, when duration of the failure detected by the fault detector means exceeds a predetermined threshold. Thereafter the power is supplied to the motor if the fault detector means does not detect a failure in the microcomputer. Alternatively, power transmission to the motor may be stopped and the microcomputer may be stalled, when duration of the failure detected by the fault detector means exceeds the predetermined threshold.